Distortion in the baseband signals recovered by a receiver caused by multi-path reception is a problem in digital television (DTV) transmissions as well as in NTSC analog television transmissions, although the distortion is not seen as ghost images by the viewer of the image televised by DTV. Rather the distortion causes errors in the data-slicing procedures used to convert symbol coding to binary code groups. If these errors are too frequent in nature, the error correction capabilities of the DTV receiver are overwhelmed, and there is catastrophic failure in the television image. If such catastrophic failure occurs infrequently, it can be masked to some extent by freezing the good TV images most recently transmitted, such masking being less satisfactory if the TV images contain considerable motion content. DTV receivers use adaptive equalizers to suppress the distortion caused by multipath reception, which equalizers are similar to those previously used in some NTSC television receivers. The adaptive equalizers are digital filters with kernel weights that can be adjusted by suitable electronics to reduce multi-path signals known as “pre-ghosts” that are received before the principal signal is received and to reduce multi-path signals known as “post-ghosts” that are received after the principal signal is received.
Several forms of adaptive equalizer are known. The adaptive equalizer can be a finite-impulse-response (FIR) digital filter formed from a several-bit-wide digital shift register a few hundred stages in length and a respective 4-quadrant digital multiplier for each stage to weight the contents of that stage by respective kernel weight for inclusion in a weighted summation. However, since many of the kernel weights are of negligible value, such a straightforward approach is wasteful of digital hardware. Adaptive equalizers currently preferred by many persons skilled in the art incorporate cascades of digital filters with specialized functions, such as the cancellation of pre-ghosts occurring a substantial number of microseconds before the principal signal, the cancellation of post-ghosts occurring a substantial number of microseconds after the principal signal, and the cancellation of so-called “micro-ghosts” that occur close in time to the principal signal but affect the amplitude and phase characteristics of the principal signal in undesired degree. The two types of digital filter earlier referred to are sometimes referred to as “ghost cancellation filters” in contradistinction to the last of these types of digital filter with specialized function being sometimes referred to as an “equalizer”, but in this specification the term “equalizer” is used in a generic sense to include all these species of digital filter.
A standard for digital high-definition television (HDTV) signals published 16 Sep. 1995 by the Advanced Television Systems Committee (ATSC) is currently accepted as the de facto standard for terrestrial broadcasting of digital television (DTV) signals in the United States of America. The data is transmitted in a succession of consecutive-in-time data fields each containing 313 consecutive-in-time data segments or data lines. The data is randomized and interleaved with a 52-data-segment (inter-segment) convolutional byte interleaver during its arrangement into the data fields. Each segment of data is preceded by a data segment synchronization code group of four symbols having successive values of +S, −S, −S and +S. The value +S is one level below the maximum positive data excursion, and the value −S is one level above the maximum negative data excursion. The segments of data are each of 77.3 microsecond duration, and there are 832 symbols per data segment for a symbol rate of about 10.76 MHz. The initial line of each data field is a data field synchronization (DFS) code group that codes a training signal for channel-equalization and multipath suppression procedures. The remaining lines of each data field contain data that have been Reed-Solomon forward error-correction coded. In over-the-air broadcasting the error-correction coded data are then trellis coded using twelve interleaved trellis codes, each a 2/3 rate punctured trellis code with one uncoded bit. Trellis coding results are parsed into three-bit groups for over-the-air transmission in eight-level one-dimensional-constellation symbol coding, which transmission is made without symbol pre-coding separate from the trellis coding procedure. Trellis coding is not used in cablecasting proposed in the ATSC standard. The error-correction coded data are parsed into four-bit groups for transmission as sixteen-level one-dimensional-constellation symbol coding, which transmissions are made without precoding.
The carrier frequency of a VSB DTV signal is 310 kHz above the lower limit frequency of the TV channel. The VSB signals have their natural carrier wave, which would vary in amplitude depending on the percentage of modulation, suppressed. The natural carrier wave is replaced by a pilot carrier wave of fixed amplitude, which amplitude corresponds to a prescribed percentage of modulation. This pilot carrier wave of fixed amplitude is generated by introducing a direct component shift into the modulating voltage applied to the balanced modulator generating the amplitude-modulation sidebands that are supplied to the filter supplying the VSB signal as its response. If the eight levels of 4-bit symbol coding have normalized values of −7, −5, −3, −1, +1, +3, +5 and +7 in the carrier modulating signal, the pilot carrier has a normalized value of 1.25. The normalized value of +S is +5, and the normalized value of −S is −5.
In the ATSC standard published 16 Sep. 1995 the data field synchronization signals in initial data segments of consecutive data fields were designed for use as ghost-cancellation reference (GCR) signals to train adaptive equalization circuitry in the DTV signal receiver. The training signal or GCR signal in the initial data segment of each data field is a 511-sample pseudo-random noise sequence referred to as “PN511 signal” followed by three 63-sample pseudo-random noise sequences referred to as “PN63 signals”. The middle ones of the PN63 signals in the field synchronization codes are transmitted in accordance with a first logic convention in the first of 313 data segments in each odd-numbered data field and in accordance with a second logic convention in the first of 313 data segments in each even-numbered data-field, the first and second logic conventions being one's complementary respective to each other. The other two PN 63 signals and the PN511 signal are transmitted in accordance with the first logic convention in each and every data field.
The middle PN63 sequence of the ATSC field synchronization code, as separated by differentially combining corresponding samples of successive field synchronization code sequences, can used as a basis for detecting ghosts. Pre-ghosts of up to −47.848 microseconds (578 symbol periods) before the separated middle PN63 sequence can be detected in a discrete Fourier transform (DFT) procedure without having to discriminate against data in the last data segment of the preceding data field. However, the post-ghosts of such data can extend up to forty microseconds into the first data segments and add to the background clutter that has to be discriminated against when detecting pre-ghosts of the separated middle PN63 sequence. Post-ghosts of up to 18.117 microseconds (195 symbol periods) after the separated middle PN63 sequence can be detected in a DFT procedure without having to discriminate against data in the precode and in the data segment of the succeeding data field. Longer-delayed post-ghosts have to be detected while discriminating against background clutter that includes data. The auto-correlation properties of the PN63 sequence are not so great that detection of longer-delayed post-ghosts is sufficiently sensitive, it appears in practice. The middle PN63 sequence of the ATSC field synchronization code provides more pre-ghost canceling capability than required in practice, but insufficient post-ghost canceling capability. While post-ghosts delayed up to forty microseconds after principal signal occur in actual practice, pre-ghosts advanced more than six microseconds before principal signal do not occur except in a poorly shielded TV receiver where a signal may be received by direct radiation as much as thirty microseconds before the same signal received via cable. Pre-ghosts preceding the principal signal by more than four microseconds are rare, according to page 3 of the T3S5 Report Ghost Canceling Reference Signals published 20 Mar. 1992 by the ATSC.
Modifying the ATSC field synchronization code, so as to place the three PN63 sequences immediately after the 4-symbol segment synchronization code and an immediately pursuant 24-symbol VSB-mode code, to be followed by the PN511 sequence and the 104-symbol gap referred to as “reserve”, would improve the ghost-separation capabilities of the separated middle PN63 sequence. Post-ghosts up to 63.364 microseconds (682 symbols duration) and pre-ghosts up to −8.455 microseconds (91 symbols duration) could be detected without data making substantial contribution to background clutter.
If one seeks to exploit the auto-correlation properties of the PN511 sequence in the ATSC DTV signal for selection of ghosts in a DFT procedure, the selection filter has to discriminate PN511 sequence and its ghosts from background clutter that includes data and the initial and final PN63 sequences. This background clutter has substantial energy, so weaker ghosts of the PN511 sequence are difficult to detect. The higher energy response of the PN511 auto-correlation filter used for ghost detection cannot be fully exploited because data and the initial and final PN63 sequences increase so much the energy of the background clutter that the filter is to discriminate against.
The training signal or GCR signal is used in many adaptive equalizers just for initializing the kernel weights, since initialization can be carried out more rapidly than is the case if adjustments of the kernel weights are made based on decisions as to the data content of currently received signal. Also, initialization using a training signal avoids the possibility of adjustments of the kernel weights stalling during least-mean-squares error calculations when a localized optimization of kernel weights is achieved that is not an ultimate optimization of those kernel weights. After initialization is accomplished, adjustments of the kernel weights are better made based on decisions as to the data content of currently received signal in a decision-directed adjustment procedure that can more rapidly adjust to changes in multi-path reception conditions, to permit tracking those changes sufficiently well that corruption of received signal is not so great as to cause frequent error in determining its data content. The need to acquire the training signal or GCR signal over the course of several data fields, in order to suppress ghosts of the principal signal sufficiently that decision-directed adjustment procedures can take over, presents a problem with ever being able initially to establish tracking of changing multi-path reception conditions.
This problem is avoided in accordance with an aspect of the invention by acquiring the GCR signal in a plurality of consecutive data segments within each data frame of the DTV signal. Acquiring GCR signal in consecutive horizontal trace intervals of an NTSC television signal is infeasible because: the information content of those lines is used to control quite directly the image traced onto the viewscreen during vertical trace period, there is no stable clock source (i.e., color burst) during the earlier horizontal trace intervals within vertical retrace period, and as a practical matter all but one of the later horizontal trace intervals within vertical retrace period is bespoken for other uses. These reasons are inapplicable to acquiring GCR signal in consecutive data segments of the DTV signal, or can be made so. There is no conformal mapping of the received data and the image traced on the viewscreen, so how GCR signal is inserted into the data stream is largely a matter of choice as long as data buffering requirements in the information pipeline do not become excessive. Measures can be provided for stabilizing the timing of sample clocks that are harmonically related to regenerated symbol clock during the consecutive data segments of the DTV signal in which the GCR signal is acquired. Since DTV is a new technology, there are no previous commercial considerations governing the use of particular data segments in the signal.
Another problem with the ghost suppression techniques previously attempted in DTV is that the effects of co-channel NTSC interference have not been given sufficient attention. The GCR signal received by a DTV signal receiver can be contaminated by artifacts of co-channel NTSC interference, particularly by standing frequencies associated with NTSC video carrier, chroma subcarrier and audio carrier. The adaptive equalizer will attempt to diminish response at those standing frequencies, which undesirably affects symbol decoding procedures. The artifacts of co-channel NTSC audio signal interference can be avoided by selective filtering in the intermediate-frequency amplifier of the DTV signal receiver, as described by A. L. R. Limberg in U.S. patent application Ser. No. 08/826,790 filed 24 Mar. 1997, entitled “DTV RECEIVER WITH FILTER IN I-F CIRCUITRY TO SUPPRESS FM SOUND CARRIER OF NTSC CO-CHANNEL INTERFERING SIGNAL”, and incorporated herein by reference.
In accordance with an aspect of the invention received DTV baseband signal and that signal as delayed 1368 symbol epochs (the duration of two NTSC horizontal scan lines) are subtractively combined in a comb filter supplying a response from which training signal is to be extracted. The artifacts of co-channel NTSC video carrier and chroma subcarrier are eliminated in this comb filter response. The data segments of the DTV signal from which the GCR signal and its ghosts are to be acquired are designed each to include the GCR signal and its complement delayed 1368 symbol epochs (the duration of two NTSC horizontal scan lines) from the original GCR signal. Accordingly, this GCR signal is reproduced with doubled energy in the response of the comb filter used to suppress the artifacts of co-channel NTSC video carrier and chroma subcarrier.